Water-absorbent resins are used for manufacture of sanitary articles such as sanitary napkins, diapers and disposable dusters, and other materials, e.g. water-holding materials used in agriculture or horticulture, materials for coagulating sludge, those for preventing dew condensation in buildings and those for removing water from oils.
Known water-absorbent resins include cross-linked carboxymethyl cellulose, partially cross-linked polyoxyethylene, hydrolyzed starch-acrylonitrile graft-copolymer, partially cross-linked polyacrylate, vinyl alcohol-acrylate copolymer, etc. The properties of conventional water-absorbent resins are greatly affected by the producing process, and there is no water-absorbent resin known in the art which possesses all the required properties including water-absorption power, water-absorbing rate, gel strength and safety. For example, conventional water-absorbent resins have the drawbacks of being low in water-absorbency, or low in gel strength after absorption of water although high in water-absorbency, leaving the gel soggy after absorption of water, namely failing to give a dry feeling, and remaining as partially dried lumps of particles after absorption of water.
It is known to improve the gel strength of water-absorbent resin after absorption of water by increasing the cross linking density of the resin. However, this method has the serious defect of reducing the water-absorbency critically required of water-absorbent resins. Further although the problem of remaining as partially dried lumps of particles can be resolved by improving the properties of particulate surface for example with a cross linking agent such as ethylene glycol diglycidyl ether, the cross linking agent is likely to irritate the human skin and thus poses a hazard problem from hygienic viewpoints when remaining in the resin.
Preferred examples of known water-absorbent resins include bead polymers prepared by polymerizing an acrylate by a reversed-phase suspension polymerization method using cellulose ester or cellulose ether as a protective colloid (Japanese Unexamined Patent Publication No. 158209/1982) or using sorbitan fatty acid ester as a dispersing agent and hydroxyethyl cellulose as a protective colloid (Japanese Unexamined Patent Publication No. 76419/1981).
However, the polymers prepared by these methods fail to satisfactorily meet the property requirements for water-absorbent resins. Particularly the gel strength of the polymers remains to be further improved. The disclosed methods give polymers which tend to readily cause blocking by standing at room temperature because of the high hydrophilic property of protective colloid used. With this shortcoming, the methods additionally require a cumbersome procedure of washing with heating the protective colloid with a solvent after the reversed-phase suspension polymerization to remove the colloid from the bead polymer prepared, hence also disadvantageous in terms of procedures.